1. Field of the Invention
The invention relates in general to a system and method for channel allocation in a multi-band wireless network, and more particularly to a system and method for channel allocation using repacking on demand (RoD) to improve call blocking probability and call handover rate.
2. Description of the Related Art
A multi-band wireless network includes base stations (BSs) and multi-band mobile stations (MSs). Many types of BSs have their radio coverage (i.e., cells) of different sizes. In the radio coverage, there are many MSs. The examples of the multi-band wireless network are Global System for Mobile Communications (GSM) 900/1800 and universal mobile telecommunications system (UMTS). In GSM 900/1800, the system consists of microcells and macrocells. Most multi-band wireless networks employ this macrocell/microcell structure. In UMTS, picocells, microcells, macrocells and hyper-cells are included in the order of cell size. The cell with larger size is overlaid with the cells with smaller sizes. This results in a hierarchical cell structure. Accordingly, the multi-band wireless network is also named multi-tier, multi-layered, multi-level, multi-mode, or hybrid cellular system. Since the macrocells/microcells structure is the most common example of multi-band wireless networks, the preferable embodiment of this invention is described based on this structure but not limited to this structure.
When a multi-band MS makes/receives a call or requests a handover, the multi-band wireless network must allocate a radio channel from either the macrocell BSs or the microcell BSs to the MS. A basic channel allocation scheme was described in the paper “Microcellular Communication Systems with Hierarchical Macrocell Overlays: Traffic Performance Models and Analysis” by Rappaport, S. S. and Hu, L. -R., Proceedings of the IEEE, Vol. 83, No. 9, pp. 1383-1397, September 1994. This basic scheme is for voice communications. In this scheme, a microcell channel is first allocated to an MS. If no microcell channel is available, a macrocell channel is then allocated to the MS. If no macrocell channel is available, the MS is blocked or forced terminated. U.S. Pat. No. 6,205,336 further applies this method to non-voice-related communication systems.
When MSs were blocked frequently, the customs will complain. Hence, the call blocking probability (i.e., the probability that a new arrival call is blocked) is a performance measure for cellular systems. In order to reduce the call blocking probability, Beraldi, R., Marano, S., and Mastroianni, C. have disclosed a method for channel allocation in the paper “A Reversible Hierarchical Scheme for Microcellular Systems with Overlaying Macrocells”, Proc. of IEEE infocom, pp. 51-58, 1996. In the method, when an MS makes/receives a call or requests a handover, channel allocation is performed as the previous basic method. Moreover, when an MS in the microcell completes its call or executes a handover to another cell, the microcell channel used by the MS is released. Then the system tries to search another call using a macrocell channel, and this call is executed a handover from the macrocell to its corresponding microcell. The above-mentioned handover from a macrocell to a microcell is called “repacking”. This increases the number of shared macrocell channels to reduce call blocking probability. However, the method causes very high call handover rate (i.e., average number that a call executes handovers) because of repacking. This increases signaling traffic and degrades communication quality.
The moving speed of the MS also affects the call handover rate. When a speaking high-speed MS moves across many microcells frequently and uses radio channels of these microcells, call handover rate is very high due to inter-microcell handover. To increase communication quality and reduce handover signaling traffic for the high-speed MS, the system had better allocate a macrocell channel than many microcell channels. Note that, to determine whether an MS is high-speed, a number of methods have been proposed. Examples are Doppler frequency measurement, U.S. Pat. Nos. 5,822,696, 6,192,245, 6,175,735, and 6,192,245. Since the measurement of MS speed is not essentially related to the invention, they will not be described further.
A method of channel allocation for a high-speed MS was disclosed in the article “Performance Analysis of Microcellization for Supporting Two Mobility Classes in Cellular Wireless Networks” by Maheshwari, K. and Kumar, A., IEEE Tran. on Vehicular Tech., Vol. 49, No. 2, pp. 321-333, March 2000. In this method, the system first allocates a macrocell channel to the high-speed MS. If no macrocell channel is available, then the system allocates a microcell channel to the MS. If no microcell channel is available, the MS is blocked or forced terminated. Note that, when a macrocell channel is reclaimed, the system finds a high-speed MS using a microcell channel for a call and this call is executed a handover from the microcell to the macrocell (if found). The handover from a microcell to a macrocell is called “taken-back”. This decreases the inter-microcell handover rate of high-speed MSs; however, this may increase one taken-back handover as soon as a macrocell channel is reclaimed.
Another method of channel allocation for a high-speed MS was disclosed in the article “Valois, F. and Veque, V Preemption Policy for Hierarchical Cellular Network”, published on 5th IEEE Workshop on Mobile Multimedia Communication, pp. 75-81, 1998. In this method, the system first allocates a macrocell channel to a high-speed MS for voice communication. If no macrocell channel is available, the system searches for a call of a slow-speed MS using a macrocell channel, this call is executed a handover from the macrocell to its corresponding microcell (if found), and then allocates the reclaimed macrocell channel to the high-speed MS. This operation is called “preemption”. If no preemption can be performed, a microcell channel is then allocated to the MS. If no microcell is available, the MS is blocked or forced terminated. However, the call blocking probability still can be improved.